Sedimentary paleoenvironment and main controlling factors of organic enrichment in source rocks of the Wenchang Formation in southern Lufeng, Pearl River Mouth Basin

doi:10.13745/j.esf.sf.2023.2.74

Abstract

Abstract:

The Wenchang Formation is the main source rock development interval in the southern Lufeng area. In order to restore the sedimentary paleoenvironment of the Wenchang period and explore the main controlling factors of organic enrichment in high-quality source rocks, the major and trace element data of 166 mudstone samples from the Wenchang Formation are analyzed. The elemental geochemical method is used to analyze the paleosalinity, paleoclimate, paleowater depth and the vertical and horizontal evolution of the paleo-oxygen phases in different subsags and different intervals of the Wenchang period for the first time in the study area. The causes of salinity anomalies in some wells and the indicative significance of triaromatic steranes and chlorella to the paleoenvironment are discussed; combined with TOC, paleoproductivity and terrestrial input changes, the main controlling factors of high-quality hydrocarbon source rocks in the study area are clarified. The results show that the water body of the Eocene Wenchang Formation is mainly freshwater during the deposition period, with slight increase in salinity in the local layer. Vertically, the salinity of the water body gradually decreases from the 4^th to the 1^st+2^nd members of the Wenchang Formation; horizontally, water salinity is highest in the Lufeng 15 subsag during deposition of the 4^th member, and relatively high in the Lufeng 13 eastern subsag during deposition of the 3^rd to the 1^st +2^nd members. It considers the change in water salinity in the Wenchang Formation is due to transgression effect, where water transgression is mainly concentrated in the Lufeng 15 subsag during deposition of the 4^th member, and expands to the eastern Lufeng 15 subsag and Lufeng 13 eastern subsag during deposition of the 3^rd member. Granodiscus and triaromatic dinoflagellate sterane cannot be used as indicators of water salinization or transgression in the study area. The change in Granodiscus content is closely related to paleoclimate, paleoproductivity and paleowater depth, but not much affected by paleosalinity changes. During deposition of the 4^th member, the paleoclimate was warm and humid, paleowater depth the largest, and paleowater reducibility the strongest; whilst in the 3^rd and 1^st+2^nd members, all three indicators decrease in values. During the deposition of the 4^th member, the water depth in the Lufeng 13 eastern subsag is the largest, followed by Lufeng 15 subsag; in the Lufeng 13 eastern subsag paleowater reducibility is the strongest, followed by Lufeng 13 western subsag. Organic enrichment in the source rocks of the Wenchang Formation is mainly controlled by paleoproductivity and paleocontinental input, followed by paleoclimate and paleowater depth. The change in paleooxygen phase has weak impact on organic enrichment. High paleoproductivity is the dominating factor controlling the formation and distribution of high-quality source rocks in the Wenchang Formation, mainly by influencing the modes of organic enrichment.

Key words: Lufeng Sag, Wenchang Formation, element geochemistry, sedimentary palaeoenvironment, Granodiscus, paleoproductivity, source rocks

CLC Number:

HE Yanbing, LEI Yongchang, QIU Xinwei, XIAO Zhangbo, ZHENG Yangdi, LIU Dongqing. Sedimentary paleoenvironment and main controlling factors of organic enrichment in source rocks of the Wenchang Formation in southern Lufeng, Pearl River Mouth Basin[J]. Earth Science Frontiers, 2024, 31(2): 359-376.

Figures/Tables 13

Fig.1 Outline map of the Pearl River Mouth Basin (a), structural unit division of Lufeng Sag (b) and comprehensive column chart of Paleogene stratum (c)

Fig.2 Calculation of paleosalinity by Couch Method in Wenchang Formation in South Lufeng Depression, Pearl River Mouth Basin

Fig.3 Paleosalinity index of 4th-1st members of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.4 Paleoclimate indexes of each subsag in the southern Lufeng area, Pearl River Mouth Basin

Fig.5 Paleodepth index of 4th-1st members of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.6 Paleo oxygen facies discrimination index of 4th-1st members of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.7 Mass chromatogram of steroids in source rocks of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.8 Geochemical indexes and paleontological changes of Wenchang Formation of Wells LF13E-6, LF13E-8 and LF13E-9 in Lufeng 13 East subsag, Lufeng Sag, Pearl River Mouth Basin

Fig.9 Correlation of paleosalinity with paleoclimate and paleowater depth in the southern Lufeng area, Pearl River Mouth Basin

Fig.10 Paleoproductivity index of 5th-1st members of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.11 Paleo terrigenous input of 4th-1st members of Wenchang Formation in the southern Lufeng area, Pearl River Mouth Basin

Fig.12 Correlation between organic carbon (TOC) content of Wenchang Formation mudstone and paleoenvironment, paleoproductivity, and input indexes of terrigenous clastic rocks in the southern Lufeng area, Pearl River Mouth Basin

Fig.13 Variation diagram of element geochemical discrimination index and organic matter abundance in Wenchang Formation of Well LF13E-10 and Well LF15-2 in the southern Lufeng area, Pearl River Mouth Basin

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